[Bugfix] Make reward CPU tests collectable without optional runtime deps

[FeathBow]

Description

Fixes #304

Importing any rm_hub submodule executes vime/rollout/rm_hub/__init__.py, which imports runtime-only deps at module level, so the reward CPU tests fail at pytest collection in lightweight environments. The chain is deeper than the issue's traceback shows: aiohttp (used by remote_rm only), vime.utils.misc / vime.utils.types (pull in torch and httpx), and sympy / pylatexenc at the top of math_utils.

Changes

• rm_hub/__init__.py: defer aiohttp and load_function to their call sites; move Sample / aiohttp under TYPE_CHECKING (same lazy-import pattern as ray in vime/utils/misc.py)
• math_utils.py: defer sympy / pylatexenc into _sympy_parse / _parse_latex / are_equal_under_sympy
• tests/test_rm_math.py: skip the two symbolic-equality tests when sympy is absent

No behaviour change.

Testing

tests/ CPU unit tests are not on CI, local results:

PYTHONPATH=$PWD python -m pytest -q tests/test_rm_math.py tests/test_rm_deepscaler.py \
  tests/test_rm_math_dapo.py tests/test_rm_f1.py tests/test_rm_gpqa.py

	lightweight env (pytest only)	full deps
main	5 collection errors (reproduces #304)	106 passed
this PR	104 passed, 2 skipped	106 passed

pre-commit run --all-files: passed.

[gemini-code-assist]

Code Review

This pull request optimizes imports across the rm_hub module by deferring heavy or optional dependencies (such as sympy, pylatexenc, and aiohttp) to local scopes or TYPE_CHECKING blocks. Additionally, it updates the test suite to conditionally skip tests that require sympy or pylatexenc if they are not installed. There are no review comments, so I have no feedback to provide.